Implantable medical devices for producing a therapeutic output to a patient are well known. Such devices include implantable devices that provide that provide an electrical stimuli to a patient, e.g., for soft tissue stimulation and/or pain control. Such devices also include implantable therapeutic substance delivery devices, e.g., devices capable of delivering a drug or other analgesia. Such implantable medical devices include implantable drug infusion pumps, implantable neurostimulators, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators and cochlear implants.
The therapeutic output, whether it is an electrical stimulus, a therapeutic substance or other therapeutic output, of a typical implantable medical device is exhaustible. That is, after having been implanted and providing a therapeutic output for a period of time, the medical device may need to be replenished in order to continue to deliver a therapeutic output. For example, a therapeutic substance delivery device may need a new supply of therapeutic substance or the therapeutic substance contained in the medical device for delivery may need to be replenished. If such a therapeutic substance is contained in a reservoir, the reservoir may need to be refilled. As another example, implantable medical devices generally need electrical power to operate. Such electrical power may be used to power electrical circuitry which operates the implantable medical device and/or may be used directly to provide electrical stimuli to a patient.
Since a typical implantable medical device has exhaustible resources, it may be important to ensure that the exhaustible resource is replenished before the resource has been exhausted in order to ensure continued, uninterrupted operation of the implantable medical device. To this end, some implantable medical devices have a mechanism that assists in determining when the exhaustible resource is close to being expended so that the resource may be replenished before exhaustion.
However, since the medical device is implanted subcutaneously in the patient, replenishment of the exhaustible resource may involve a surgical procedure to refill, recharge or even explant and re-implant the medical device. Thus, replenishment of an exhaustible resource in an implanted medical device may not be an act that is taken lightly.
The time at which an exhaustible resource in an implanted medical device is replenished can be extremely important. If the exhaustible resource is not replenished soon enough, the implanted medical device may stop functioning which could lead to deleterious and, possibly, disastrous, results. If, however, the exhaustible resource is replenished too soon, a surgical procedure may need to be performed before the surgical procedure is needed adding unnecessary risk, inconvenience and cost and, perhaps, requiring more surgery than otherwise would optimally be required.
It is typical to utilize a worst case scenario for determining when to replenish an exhaustible resource in an implanted medical device. Since it may be imperative not to allow an implanted medical device to completely exhaust an exhaustible resource, and possibly allow the implanted medical device to cease to perform its therapeutic function. The worst case scenario may depend upon variables in the construction and operation of the implanted medical device.
For example, if a chemical battery is an exhaustible resource of an implanted medical device, batteries employed in the construction of the medical device may have a tolerance in capacity. If so, it is typical to assume that the battery of any particular implantable medical device contains a battery having the minimum possible capacity within the tolerances allowed. Further, it may be typical to assume the most unfavorable tolerances of power consumption of the components of the implantable medical device. Still further, it may be typical to assume that the implanted medical device will generate the maximum possible stimuli. All of these assumptions taken together can ensure that the implanted medical device is replenished before it ceases to function. However, all of these assumptions taken together may also dictate a replenishment time which is substantially earlier than may be actually required.
As another example, if the implantable medical device is adapted to deliver a therapeutic substance to a patient, the amount of therapeutic substance actually delivered to the patient may depend upon many construction variables, including motor tolerances, and may also depend upon the number and amount of patient boluses delivered by the implanted medical device. It may be typical to assume the most unfavorable tolerances for medical device components and to assume the maximum number and volume of patient boluses in order to ensure that the implanted medical device is replenished before it ceases to function. Again however, all of these assumptions taken together may also dictate a replenishment time which is substantially earlier than may be actually required.
Accordingly, there is a need for an implantable medical device and method that addresses the above issues.